1. Technical Field
The present invention relates to a disk substrate molding apparatus, a disk substrate molding method and a disk substrate molding die.
2. Background Art
A disk-shaped optical recording medium with a substrate made of resin has been put into practical use as a large-capacity, low-cost information recording medium. For example, as this kind of optical recording medium, there are a CD (compact disk), a DVD (digital versatile disk), and a BD (Blu-ray disk). The resin substrate of this optical recording medium is manufactured with the injection molding method from the perspective of shortening the manufacturing time. With this molding method, molten resin is filled in a cavity formed within a die, and the resin is solidified and subsequently removed from the die.
FIG. 13 shows an example of a conventional die. In FIG. 13, reference numeral 101 shows a fixed-side die, and reference numeral 102 shows a movable-side die. The space created by closing the fixed-side die 101 and the movable-side die 102 is a cavity 103. The cavity 103 is where the molten resin is filled and molded into a substrate.
The fixed-side die 101 comprises a sprue bush 104 and a fixed-side mirror plate 105, and these are disposed in order from the inner circumferential side. The sprue bush 104 is a member for causing the molten resin extruded from a nozzle (not shown) of the injection molding machine to flow into the die. A water passage 106 is formed on the fixed-side mirror plate 105, and the temperature of the fixed-side mirror plate 105 is controlled by causing temperature controlled water to circulate in the water passage 106. The sprue bush 104 and the fixed-side mirror plate 106 are joined with the fixed-side plate 107. The fixed-side mirror plate 105 is fitted into the outer circumferential ring 108 and joined via a spring 109. The outer circumferential ring 108 is a member for deciding the outer circumferential end shape of the substrate.
The movable-side die 102 comprises an ejector pin 110, a cut punch 111, an ejector 112, a stamper holder 113 and a movable-side mirror plate 114, and these are disposed in order from the inner circumferential side. The cut punch 111 is a member for forming an inner hole in the molten resin that was filled in the cavity 103. The ejector pin 110 and the ejector 112 protrude from the mirror disk 114 after the molten resin is solidified. The ejector pin 110 and the ejector 112 are used for removing, from the movable-side die, the substrate part molded from the molten resin and the unneeded part in a mutually separated state. The stamper holder 113 is used for retaining the stamper 115 with the movable-side mirror plate 114. The stamper 115 is used for forming fine concavity and convexity on the substrate surface, and is generally made of nickel. The movable-side mirror plate 114 is also formed with a water passage 116 as with the fixed-side mirror plate 105. The temperature of the movable-side mirror plate 114 is controlled by causing temperature controlled water to circulate in the water passage 16. Note that, in FIG. 13, reference numeral 117 is a movable-side plate, and the movable-side mirror plate 114 is joined with the movable-side plate 117.
When molding a resin substrate with the injection molding method, even in cases where the thickness of the cavity is molded in a uniform width from the inner circumference to the outer circumference, the vicinity of the outer circumferential end of the molded resin substrate tends to become thicker than the inner circumference or the middle circumference. This phenomenon is referred to as a ski jump or an edge wedge.
FIG. 14 is a cross section of a resin substrate molded with the injection molding method, and FIG. 15 is a conceptual diagram of a resin substrate where the edge wedge of the outer circumferential part is illustrated with emphasis. Here, a resin substrate with a diameter of 120 mm and thickness of 1.2 mm is taken an as example, and the explanation is provided based on this substrate. The thickness of the resin substrate gradually increases from a position that is 3 mm inward from the outer circumferential end toward the outer circumferential end, and the increment is also increasing. Although the maximum amount of swelling of the edge wedge depends on the molding conditions and the die, it is generally within the range of 10 μm to 40 μm.
As causes of the edge wedge, for example, pressure is applied to the resin as a result of the molten resin filled in the cavity coming in contact with the member forming the outer circumferential end of the substrate and thereby causing the density to increase, and the resin within the cavity is cooled from the circumferential side that is more outward than the inner circumference and middle circumference, in particular from the outer circumferential end, and solidification progresses initially from such outer circumferential end.
In order to record information of a large capacity without changing the size of the substrate in an optical recording medium, it is necessary to reduce the size of the beam spot for recording and reproducing information on the optical recording medium. Thus, in order to increase the recording capacity, the distance (working distance) from the pickup that focuses the beam to the optical recording medium must be narrowed. This means that the maximum angle of incidence of the beam to fall incident on the optical recording medium will increase. Meanwhile, with a magnetic recording medium of a hard disk device, information is recorded, reproduced and erased using a flying head that floats in the air. In order to record information of greater capacity in a magnetic recording medium, the distance between the flying head and the magnetic recording medium needs to be narrowed since the magnetic field emitted from the head generated in a narrower area needs to reach the magnetic recording medium.
With optical recording mediums, research is being conducted for reducing the size of the beam spot by combining a solid immersion lens (SIL) with a condenser lens. With this technology, in order to obtain a capacity of 500 GB with an optical recording medium having a diameter of 120 mm, it is said that the distance from the solid immersion lens to the optical recording medium needs to be 50 nm or less. Moreover, research is also being conducted for reducing the floating distance of the flying head of a magnetic recording medium. In order to obtain a capacity of 500 GB with a magnetic recording medium having a diameter of 90 mm, it is said that the floating distance needs to be 10 nm or less. Meanwhile, with BD (Blu-ray disk) that has been put into practical use, the distance from the condenser lens to the optical recording medium is 0.3 mm.
When the distance from the pickup or head to the information recording medium is narrowed as described above, there is a possibility that the information recording medium and the pickup or head may collide if the unevenness on the surface of the information recording medium is large. Moreover, even if they do not collide, since the distance between the information recording medium and the pickup or the head will become inconsistent, the recording, reproduction and erasing of information with the information recording medium will be affected. If there is an edge wedge in the vicinity of the outer circumferential end of the resin substrate molded with the injection molding method, information cannot be recorded in the vicinity of the outer circumferential end of the information recording medium.
Moreover, as the distance from the pickup or the head to the information recording medium is narrowed, information cannot be recorded in a broader range at the outer circumference of the information recording medium. This is because, since the pickup or the head has a certain degree of width, the distance from such portion to the recording medium becomes smaller than the tolerance, and will affect the recording, reproduction and erasing of information with the information recording medium.
Moreover, since the distance of the circle of the outer circumferential part is longer in comparison to the inner circumferential part, even if it is the same radial distance, the outer circumferential part can record more information than the inner circumferential part. Accordingly, it is essential to reduce the edge wedge in the vicinity of the outer circumference of the resin substrate in order to increase the recording capacity of the information recording medium.
Meanwhile, if it is possible to eliminate the edge wedge in the vicinity of the outer circumference of the resin substrate manufactured with injection molding, a recording medium with a larger capacity than the existing BD can be manufactured by applying existing technologies.
As a method for eliminating the edge wedge of the resin substrate after performing injection molding, cutting or grinding can be performed. Specifically, after molding the resin substrate in a slightly larger size, the portion containing the edge wedge of the outer circumferential is cut, and the inner circumferential part thereof is used as the resin substrate. Otherwise, without changing the outer diameter of the resin substrate, the swelling of the actual edge wedge of the resin substrate can be removed by cutting or grinding. Nevertheless, the method of removing the edge wedge after performing injection molding results in increased manufacturing processes.
Thus, as a method of reducing the edge wedge of the resin substrate using the injection molding method that does not require the foregoing after processing, known is a method of providing a heat preventing mechanism to the die and delaying the cooling and solidification at the outer circumferential end of the resin substrate. For example, disclosed are technologies of providing, as the heat preventing mechanism, a resistance heater, a film resistance heater or an induction heater to an outside retainer (corresponds to 108 in FIG. 13) which forms the outer circumferential end of the resin substrate with the die, or circulating heated water or oil in the outside retainer (for example, refer to Patent Document 1). Moreover, as other methods of reducing the edge wedge, as shown in FIG. 16, disclosed are technologies of increasing the thickness of the outer circumferential end of the stamper 115 mounted on the die, or, as shown in FIG. 17, narrowing the gap in the cavity in the vicinity of the outer circumferential end of the resin substrate by retaining the stamper 115 so that the outer circumferential end of the stamper 115 is warped toward the cavity side (refer too Patent Document 1 and Patent Document 2).
Nevertheless, the conventional methods entail the following problems.
Foremost, with a die provided with a heat preventing mechanism as with Patent Document 1, although it is effective in reducing the amount of the edge wedge, it is unable to completely eliminate the edge wedge.
Moreover, with the method of increasing the thickness of the outer circumference of the stamper as with Patent Document 1, not only is it troublesome, the reproducibility is low since the thickness tends to change for each stamper.
Moreover, with a die in which the vicinity of the outer circumferential end of the stamper is deformed as with Patent Document 2, since the stamper is subject to repeated deformation of bending and returning to its original form each time one resin substrate is molded, the stamper wears easily and its life is shortened.